It has recently been suggested that microporous hollow fibers (MHF) may be employed in a liquid membrane separation technique whereby feed and sweep gases flow through the lumens of two different sets of hydrophobic MHF (designated feed-fibers and sweep-fibers, respectively), while a liquid on the shell side of the MHF serves as the membrane. See generally, Majumdar et al, "A New Liquid Membrane Technique for Gas Separation", AICHE Journal, vol. 34, No. 7, pages 1135-1145 (1988), and Sengupta et al, "Separation of solutes from Aqueous solutions by Contained Liquid Membranes", AIChE Journal, vol. 34, no. 10, pages 1698-1708 (1988), the entire content of each being expressly incorporated hereinto by reference. This so-called "contained liquid membrane" (CLM) technique is reported to have several advantages over conventional immobilized liquid membrane (ILM) separation technology.
For example, conventional ILM technology typically requires periodic replacement of the immobilized membrane liquid due to solute saturation, depletion and/or contamination (depending upon the type of separation being conducted). As a result, conventional ILM technology is typically only limited to batch separation processing. However, since the membrane liquid according to the recently proposed CLM technique is physically present in the shell-side of a separation module, it may be replenished and/or replaced more or less continually thereby allowing separation processing to be accomplished on an essentially continuous basis.
Modules for performing CLM separation processes typically include a bundle of hollow fiber membranes divided approximately equally into a set of feed-fibers (through which the feed fluid flows), and a set of strip-fibers (through which the strip fluid flows). The MHF bundle is physically housed within a module case of desired size and configuration such that the lumens of the feed- and strip-fibers are in fluid-communication with supply and discharge ports of the module case associated with the flow of feed and strip fluids, respectively. In this manner, a cocurrent or countercurrent gas flow through the respective sets of feed- and strip-fibers within the module case may be established.
Theoretically, when performing CLM separations, each of the feed-fibers should be in an immediately adjacent non-contacting relationship to a respective one of the strip-fibers so that the distance therebetween is filled with the membrane liquid. According to this ideal configuration, therefore, a theoretical minimum effective membrane thickness (EMT) is established whereby the closest packing of the feed and strip fibers is achieved so that the distance therebetween is minimized. However, conventional module manufacturing techniques fall far short of the theoretical minimum EMT since individual feed-fibers cannot exactly and reliably be interposed with individual strip-fibers. As a result, groupings of feed-fibers will reside in the module adjacent to groupings of strip-fibers thereby significantly increasing the module EMT over the theoretical minimum value.
It is towards providing solutions to the above problems that the present invention is directed. Broadly, therefore, the present invention is directed to modules containing hollow fiber membranes adapted to being used for contained liquid membrane separations which exhibit effective membrane thicknesses which are closer to the theoretical minimum value than can be obtained using conventional membrane manufacturing techniques.
More specifically, the present invention is directed to modules having superposed fabric sheets in which hollow fiber membranes are disposed in the fabric's warp-wise direction. The warp-wise hollow fiber membranes in one of the fabric sheets can therefore be dedicated as feed-fibers through which a feed fluid flows, whereas the hollow fiber membranes in the other of the fabric sheets can be dedicated as strip-fibers through which a strip fluid flows.
The superposed fabric sheets are also co-pleated with one another in the warp-wise direction so that the respective folds of the fabric sheets will be nested with one another. In this manner, the feed- and strip-fibers of the respective fabric sheets are alternately disposed within the module and are adjacent to strip- and feed-fibers, respectively. As a result, reduced EMT values as compared to conventional CLM modules may be obtained.
Preferably, the fabrics employed in the present invention are woven fabrics in which the warp-wise hollow fiber membranes are interwoven with weft-wise monofilamentary fibers. However, other fabric forms may also be utilized according to the present invention--for example, a knitted structure in which the hollow fiber membranes are inserted as a filling.
The weft-wise fibers serve to provide structural support for the warp-wise hollow fiber membranes so as to maintain fiber-to-fiber parallelism between the hollow fiber membranes in the lengthwise direction of the module. In addition, the weft-wise fibers serve as "spacers" which minimize (if not essentially eliminate) contact between the microporous hollow feed- and strip-fibers as well as imparting self-centering functions to the hollow fiber membranes in the superposed fabric layers. These functional characteristics of the weft-wise fibers further minimize the EMT value of the module (i.e., the module EMT approaches the theoretical value).
Further aspects and advantages of this invention will become more clear after careful consideration is given to the detailed description of the preferred exemplary embodiments thereof which follows.